The use of directly fired combustors for heating purposes even at high temperatures up to in the order of 1000 degrees C. is common in the industry. The heat loss experienced by conventional directly fired combustors is generally directly proportional to their operating temperatures resulting in a high temperature causing a substantial loss of heat in turn implying an increased fuel consumption. Very high temperatures also produce fatigue in the materials used in the surroundings of a combustor. A lower process temperature requires an intensification of the speed at which convection heat reaches its intended source. While conventional combustors often require specific medium supply pressures a variation therein affects the efficiency of these combustors as they do not have the ability to accommodate a variation in medium supply pressures. The running of combustors at lower temperatures and increased efficiencies further has a beneficial effect on the release of greenhouse gases and urban nitro-oxides. It is, amongst others, an object of this invention to address the situations mentioned for improving combustor usefulness and efficiency.